This invention relates to operation systems for hoisting devices of construction machinery such as cranes, and more particularly it is concerned with an operation system for a hoisting device of the type comprising a clutch mechanism, a foot brake mechanism and an automatic brake mechanism.
One type of operation system for a hoisting device of a crane known in the art comprises a clutch mechanism for connecting and disconnecting a hydraulic motor and the hoisting device, a foot brake mechanism for applying a foot brake to the hoisting device by the control of an operator, and an automatic brake mechanism for applying an automatic brake to the hoisting device.
The clutch mechanism comprises a clutch cylinder and a spring operative such that when the clutch cylinder is in communication with a reservoir the spring performs clutch disconnection and when the clutch cylinder is in communication with a hydraulic fluid source the clutch cylinder performs clutch connection with a hydraulic fluid supplied from the hydraulic fluid source. Thus, it is possible to perform clutch connection and clutch disconnection by controlling the introduction of the hydraulic fluid into the clutch cylinder.
The foot brake mechanism functions as a positive brake system which applies the brake to the hoisting device only when the operator manipulates it.
The automatic brake mechanism comprises a brake cylinder and a spring operative such that when the brake cylinder is in communication with the reservoir the spring applies the brake and when the brake cylinder is in communication with the hydraulic fluid source the brake cylinder releases the brake with a hydraulic fluid supplied from the hyraulic fluid source. Thus, the automatic brake mechanism in which the brake cylinder is brought into communication with the reservoir when an operation lever for the hoisting device is in a neutral position and the brake cylinder is brought into communication with the hydraulic fluid source when the operation lever is in a non-neutral position functions as a negative brake system which normally applies the brake to the hoisting device and releases the brake when the operation lever is moved for lifting or lowering operation.
One example of the operation system for the hoisting device of such a construction is disclosed in Japanese Utility Model Application Laid-Open No. 110971/79.
Thus, in such an operation system, it is possible to perform a negative brake opration by turning on the automatic brake mechanism and the clutch mechanism. In the negative brake operation, it is possible to carry out holding of a suspended load by using the automatic brake and lifting and power lowering of the suspended load by releasing the automatic brake. By turning off the automatic brake mechanism, it is possible to perform a positive brake operation by utilizing the foot brake mechanism. In this case, it is possible to perform lifting and power lowering of a suspended load by turning on the clutch mechanism and it is possible to perform free fall of the suspended load by turning off the clutch mechanism.
The operation system for the hoisting device of the prior art has suffered the disadvantage that on-off control of the automatic brake mechanism and on-off control of the clutch mechanism should be performed by the operator himself independently and separately. Thus, the operator has to perform the aforesaid on-off controls in addition to the controls of the operation lever for the hoisting device and the foot brake mechanism, with a result that the operator has to perform a complicated operation and misoperation tends to occur.
Also, in the operation system of the prior art, it sometimes happens to hold a suspended load by a hydraulic brake comprising the hydraulic motor and a counterbalance valve connected to the circuit of the hydraulic motor by turnig off the automatic brake mechanism and turning on the clutch mechanism. When this is the case, lowering of the suspended load would occur due to leaks of the oil from the hydraulic motor.